Bacteria communicate via a phenomenon termed quorum sensing (QS) in which they secrete chemical signal molecules, termed autoinducers, into their surrounding environment. The concentration of these signal molecules increases locally as a result of increasing cell density, and upon reaching a threshold level (when the population is ‘quorate’) the population activates a coordinated cellular response such as the production of virulence factors and growth as a biofilm community.
Pseudomonas aeruginosa (or simply P. aeruginosa) is a ubiquitous Gram-negative bacteria that is responsible for many opportunistic and nosocomial infections and chronic infection by P. aeruginosa is the leading cause of death of cystic fibrosis patients. P. aeruginosa has three main QS systems. The first two QS systems, LasR-LasI and RhlR-RhlI, are based on the LuxR-LuxI homologues of Vibrio fischeri, which makes use of acyl homoserine lactone (AHLs) as signal molecules. The AHL synthases are LasI and RhlI, which produce N-(3-oxododecanoyl)-L-homoserine lactone (OdDHL) and N-butanoylhomoserine lactone (BHL), respectively. The receptor for OdDHL is the LasR protein, while the receptor for BHL is the RhlR protein.
Together, the las and rhl QS systems regulate a host of virulence factors such as exoproteases (an example being elastase), siderophores, and toxins. The third signaling system utilizes another kind of signal molecule, 2-heptyl-3-hydroxy-4-quinolone, that has been termed the Pseudomonas quinolone signal (PQS) and is able to affect the expression of Las and Rhl-controlled genes. LasR is an attractive target for QS inhibition as LasR controls the other QS circuits (namely Rhl and PQS) within the P. aeruginosa hierarchy. The las and rhl systems are at the top and bottom of the hierarchy respectively, while the PQS system intervenes between them.
As QS controls the expression of multiple virulence factors in different bacteria, blocking of QS would be vital in attenuating the virulence of pathogenic bacteria. During the last decade, the QS system has been proposed as a target for developing next generation antimicrobial agents. The rationale for interrupting bacterial communication rather than inhibiting growth is because QS inhibitors (for short, QSIs), by targeting non-essential processes, are shown to not exert strong selective pressure for the evolution of resistance mechanisms as compared to the conventional growth-inhibitory compounds.
The conventional approach to identify QSIs is by using biosensor systems which often fuse a QS-regulated promoter to the lux, gfp or lacZ reporter genes. A wide range of QSIs was identified by the use of these biosensor systems. However, QSIs identified through the use of biosensors might not be target-specific and have some potential risk in their application. QS regulation is integrated into the complex bacterial regulation networks which also include nucleotide signaling (e.g. cAMP and c-di-GMP), iron signaling, phosphate signaling, and so on. Thus, QSIs identified through the use of biosensor systems might actually target other regulators which may also affect QS. This brings the risk that these QSIs might be able to induce virulence factors regulated by other regulation networks even though they can inhibit QS.
In contrast to the conventional lab-based screens, some have utilized a computer-based approach to drug screening known as structure-based virtual screening (SB-VS). SB-VS can be defined as a method to computationally screen large compound libraries for molecules that bind targets of known structure, and then test experimentally those predicted to bind well. Recent successes of this approach include: inhibitors against the apoptosis regulator Bcl-2, Hsp90, G-protein coupled receptors and metalloenzymes.
With the recent availability of crystal structures of bacteria QS receptor proteins such as LasR of P. aeruginosa and TraR of Agrobacterium tumefaciens, SB-VS has become a viable option for QSI discovery.